Generally, the ball screw unit basically comprises a screw provided with a helical ball rolling groove on the outer peripheral surface thereof, a nut provided on the inner peripheral surface thereof with a helical load rolling groove held opposite to the ball rolling groove of the screw and having the same lead angle as the latter and a number of balls interposed between the load rolling groove and the ball rolling groove so as to roll while bearing a load and wherein an endless track for causing the balls to circulate therethrough is formed by constructing a nut assembly by fixing a ball circulating tubular body or a pair of covers to the nut.
Conventionally, there have been known several methods for forming the above-described ball endless track.
For example, regarding the tube type ball screw unit, there is known a tube system (Japanese Unexamined Utility Model Publication No. S49-64,672) in which as shown in FIGS. 40 and 41, ball tubes(f)(ball circulating tubular bodies) each provided on both ends thereof with tongues (g) are passed between the starting end and the terminating end of a load zone formed by a ball rolling groove(b) of a screw(a) and a load rolling groove(d) of a nut(c) such that each of balls(e) rolling within the ball rolling groove(b) of the load zone is picked up or fed into the groove (b) by the tongues (g) projecting into the groove (b) so that each of the balls(e) coming out from the terminating end of the load zone is circulated again into the starting end of the load zone through the ball tubes (f).
According to this tube system, however, when each of the balls(e) moves from the terminating end of the load zone of the endless track to the no-loaded zone thereof, the ball(e) runs against the tongue(g) of each of the tubes (f) so that the rolling ball(e) is picked up into the ball tube(f) forming the no-load zone and is then circulated again into the starting end of the load zone.
Further, as regards the side cover type ball nut and screw unit, there is known a side cover system disclosed in the U.S. Pat. No. 4,148,226 (entitled "Ball nut and screw assembly")in which as shown in FIGS. 42 through 44, a ball return hole(h) is drilled in a nut(c) along an axial direction of the outer peripheral solid portion thereof and a pair of side covers (k) are respectively fixed to both ends of the nut(c) through spacers(i) so that the progressing direction of each of balls(e) coming out of the terminating end of a load zone formed by a ball rolling groove (b) of a screw(a) and a load rolling groove(d) of the nut(c) is inclined by each of the side covers (k) and each of the spacers(i) thereby guiding the ball(e) from the terminating end of the load zone up to the outer cylindrical surface of a screw (a). Further, a change direction path(j) for guiding the ball(e) up to the ball return hole(h) of the nut(c) is formed in each of the side covers thereby forming a ball endless track.
However, this system is also constructed such that when the ball(e) moves from the terminating end of the load zone of the ball endless track to the no-load zone, the ball (e) runs against the side wall of the change direction path (j) inclined with respect to the ball progressing direction at the terminating end of the load zone, that is, from the tangential direction along the lead angle of the load rolling groove(d), so that the rolling ball(e) is picked up into the change direction path(j) forming the no-load zone and is circulated again to the starting end of the load zone.
Therefore, in the cases of the above-described conventional ball screw units, when each of the balls rolling in the load zone is picked up into the no-load zone or the balls in the no-load zone are fed to the load zone, the presence of the tongue (g) for changing the progressing direction of the rolling balls is indispensable so that the smooth circulation of the balls is impaired and as a result, the uniform and stabilized rotation of the ball screw unit is impaired, the working life of the unit is shortened or noises generate.
Further, in Unexamined Published Japanese Utility Model Application No.H5-27,408, there is disclosed a ball screw unit wherein as shown in FIG. 45 thereof, a flexible belt-like retainer (i) is slidably inserted into a ball circulation path (h) provided with a ball circulating pipe (a ball tube (f)) so as to extend over the entire length of the path (h) and a number of ball pockets (not shown) are mounted on the retainer (i) at predetermined intervals over the entire length thereof so that balls(e) are rotatably retained within the pockets, respectively, whereby the adjoining balls do not come into contact with each other, the ball screw unit can have an excellent operation characteristic free of torque dispersion and can allow the balls to smoothly circulate through the ball circulation path(h).
However, in the case of such ball screw unit, the belt-like retainer(i) assembled into the ball circulation path(h) is inevitably twisted and bent and it is necessary to absorb such twisting and bending of the retainer (i) within the ball circulation path(h) so that although the idea of "providing a ball screw unit having an excellent operation characteristic by preventing the adjoining balls from coming into contact with each other by the belt-like retainer assembled into the ball circulating path" may be accepted as reasonable, there is actually no suitable means as to how to absorb the twisting and bending of the belt-like retainer (i) within the ball circulating path (h) and it is the actual situation that such idea has not been materialized as a complete product.
Thus, in the cases of the conventional ball screw units, irrespective of whether or not they employ the system for forming a ball endless track, when each of the balls rolling in the load zone is picked up into the no-load zone or each of the balls in the no-load zone is fed into the load zone, it becomes necessary to provide a means for changing the progressing direction of the balls so that the smooth circulation of the balls is inevitably impaired which results in that the uniform and stabilized rotation of the ball screw unit is impaired, the working life of the unit is shortened or noises generate.
Therefore, as a result of their intuitive investigation of means for fundamentally solving the above-described problems, the present inventors have completed the present invention by finding out the following fact. That is, when a ball coupler is formed by making use of a plurality of flexible coupling members having the function of rotatably retaining a number of balls in their aligned state, a guide groove for guiding the coupling members of the ball coupler is formed in a no-load zone of a ball endless track, the ball coupler is moved by each of the balls in a load zone of the endless track while the balls are guided to move in the no-load zone of the endless track by the coupler and guide sections for guiding the coupling members of the ball coupler along the progressing direction of the balls moving in the load zone are respectively provided at both ends of the no-load zone at least near the load zone, the delivery and reception of the balls between the load zone and the no-load zone of the endless track are performed smoothly to thereby achieve the smooth circulation of the balls within the endless track.
Accordingly, an object of the present invention is to provide a ball screw unit which comprises a screw, a nut assembly screw-fitted with the screw through a ball and having a ball endless track and a ball coupler assembled into the endless track of the nut assembly and which makes smooth the delivery and reception of the balls to be performed between a load zone and a no-load zone of the endless track thereby achieving the smooth circulation of the balls.
Another object of the present invention is to provide a tube type ball screw unit including a nut assembly comprising a nut having a load rolling groove forming a load zone of a ball endless track and a ball circulating tubular body having a no-load ball path forming a no-load zone of the ball endless track whereby the delivery and reception of the balls to be performed between the load zone and the no-load zone of the ball endless track can be performed smoothly thereby achieving the smooth circulation of the balls.
Still another object of the present invention is to provide a side cover type ball screw unit which includes a nut assembly comprising a nut having a load rolling groove forming a load zone of a ball endless track and a no-load ball path forming a no-load zone of the ball endless track and a pair of side covers respectively attached to both ends of the nut and having change direction paths forming the no-load zone of the ball endless track, and which makes smooth the delivery and reception of the balls between the load zone and the no-load zone of the endless track to thereby achieve the smooth circulation of the balls.